This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to oil filled process transmitters such as pressure transmitters having capacitive pressure sensors.
Process instruments are used to monitor process parameters, such as pressure, temperature, flow and level, of process fluids used in industrial processes. For example, process transmitters are typically employed in industrial manufacturing facilities at multiple locations to monitor a variety of process parameters along various production lines. Process transmitters include sensors that produce an electrical output in response to physical changes in the process parameter. For example, pressure transmitters include capacitive pressure sensors that produce an electrical output as a function of the pressure of a process fluid, such as water lines, chemical tanks or the like. Each process transmitter also includes transmitter electronics for receiving and processing the electrical output of the sensor so that the transmitter and process parameter can be monitored locally or remotely. Locally monitored transmitters include displays, such as LCD screens, that show the electrical output at the site of the process transmitter. Remotely monitored transmitters include electronics that transmit the electrical output over a control loop or network to a central monitoring location such as a control room. Configured as such, the process parameter can be regulated from the control room by including automated switches, valves, pumps and other similar components in the control loop.
A typical capacitive pressure sensor used in a pressure transmitter includes a fixed electrode plate and a movable electrode plate, which typically comprises a flexible sensing diaphragm. The sensing diaphragm is connected to the process fluid through a simple hydraulic system that communicates the process fluid pressure to the sensor. The hydraulic system comprises a sealed passageway in which the sensing diaphragm is positioned at a first end, and a flexible isolation diaphragm is positioned at a second end to engage the process fluid. The sealed passageway is filled with a precise amount of hydraulic fluid that adjusts the position of the sensing diaphragm as the process fluid influences the isolation diaphragm. As the pressure of the process fluid changes the position of the sensing diaphragm changes, resulting in a change in capacitance of the pressure sensor. The electrical output of the pressure sensor is related to the capacitance and thus changes as the process fluid pressure changes.
The capacitance of the pressure sensor is controlled by three main factors: the surface area of the electrode plates, the distance between the electrode plates and the magnitude of the dielectric constant of the matter between the electrode plates, typically the hydraulic fluid. It is generally desirable to produce pressure sensors as small as possible such that they can be used in a greater number of applications. The lower limit on the spacing between the electrodes is limited by the ability of the capacitor to function properly. The lower limit on the surface area of the plates is dictated by the necessity of the pressure sensor to generate a signal having a minimum strength compatible with the transmitter electronics. Spacing and surface area parameters are also limited by manufacturing tolerances. The dielectric constant of the matter between the plates is limited by the types of fill fluids that are compatible with the hydraulic system. There is, therefore, a need for capacitive-based pressure sensors having higher capacitances.